I haven't had personal experience with broadside-coupled differential pairs.
I do know a couple of people who got burned with them, but not why they got
into problems.

The fab houses have tended to discourage broadside because of the number of
manufacturing variables they have to control. On the edge-coupled boards we
did the vendors were able to hold the impedance within 1 ohm over hundreds
of paths and a number of boards, which I thought was fabulous.

I think there was a DesignCon 2000 paper given on differential pairs that
concluded that if you matched the parameters correctly for each case there
was actually no measurable difference. One of the diff pair gurus on the
reflector will no doubt have pointers and/or opinions.

One thing that might be a factor: the common connectors are all edge-coupled
in design, so I would guess that theoretically in a broadside-coupled PCB
system you would have to change propagation modes going through the
connectors. Does this make a difference? I do not know. I seem to recollect
from 30 years ago that in rectangular wave guide E-plane bends were worse
than H-plane bends (or maybe vice-versa).

I used edge-coupled because AMP recommended it for their HS-3 connectors and
gave very detailed specifications for routing through pin fields, etc. This
made it easier to communicate with the PCB designer ("do it like the
picture").

The use of stripline was because of the number of channels I had to do. It
turned out that I could fully interconnect the diff pair part of the board
with 4 layers of stripline pairs and no vias. That would not have been
possible using only the surface layers. (I don't think I said microstrip
anywhere, but if I did I meant stripline. Very dyslexic morning.)

I was lucky in one respect: I did the back plane before the plug-in cards
were designed, so I had "dibs" on pinouts that made the back plane route
with no crosses. Actually, it turned out that this was good for the plug-in
part of the system as well. It certainly showed the advantage

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